1. Field of the Invention
The present invention relates to a near-field terahertz wave detector having a spatial resolution beyond the diffraction limit in terahertz waves.
2. Description of the Related Art
In the present invention, the term “terahertz wave” means an electromagnetic wave whose frequency is in the range of 1 to 10 THz (1 THz=1012 Hz), in other words, whose wavelength is in the 0.03 mm to 0.3 mm submillimeter-wave to far-infrared region.
The terahertz wave is expected to be applied in a wide range of fields extending from basic research such as radio astronomy, materials science, and biomolecular spectroscopy to practical applications such as security, information communication, environment, and medical care.
Particularly, an imaging technique using terahertz waves (hereinafter, referred to as “THz imaging technique”) is expected to be a powerful measurement tool in the fields such as a human body test, materials evaluation, and the like, since the terahertz waves have characteristic properties such as moderately passing through materials opaque to visible light and having photon energy that belongs to an important energy range of meV for various materials.
The terahertz wave, however, is an electromagnetic wave having a frequency band between light such as infrared radiation, visible light, and UV radiation (frequency: 1013 to 1015 Hz) and a radio wave (frequency: 103 to 1012 Hz), which leads to a problem that it is impossible to directly use the existing techniques of optics and electronics to the terahertz wave.
Particularly in the THz imaging technique, there is a problem that the spatial resolution is relatively low in comparison with the visible light since the wavelength of the terahertz wave is relatively long in comparison with the visible light and the spatial resolution is limited to the order of one half of the wavelength due to the diffraction limit.
Accordingly, the use of a near-field wave is considered as a means for implementing THz imaging having high spatial resolution beyond the diffraction limit.
The term “near-field wave” means light extremely thinly clinging to the surface of an object and has a property of not propagating in a space like the normal light. For example, it is known that totally-reflected light oozes out of the boundary surface into the air at a total reflection point in a prism. The light oozing into the air is called a near-field wave or an evanescent wave.
Although a standard optical microscope magnifies light from an object using a lens, the magnification is limited by the wavelength of light (the wavelength of visible light is in a range of approx. 0.38 to 0.77 μm) and the standard optical microscope is capable of resolving only up to approx. 0.5 μm. It is referred to as “diffraction limit” of light waves.
The near-field wave, however, does not propagate in a space like the normal light and therefore is not affected by the diffraction limit. Therefore, it is possible to achieve a microscope having a resolution beyond the diffraction limit by using the near-field waves.
Nonpatent Documents 1 to 3 have already disclosed examples of the THz imaging using the near-field waves.
[Nonpatent Document 1]    S. Hunsche et al., “THz near-field imaging,” Optics Communications 150 (1998) 22-26
[Nonpatent Document 2]    Wang et al., “Antenna effects in terahertz apertureless near-field optical microscopy,” Appl. Phys. Lett., Vol. 85, No. 14, 4 Oct. 2004
[Nonpatent Document 3]    Chen, Kerstingm and Cho, “Terahertz imaging with nanometer resolution,” Appl. Phys. Lett., Vol. 83, No. 15, 13 Oct. 2003
For the visible and near infrared regions, there are well established techniques utilizing a small aperture or a small scatterer with a tapered optical fiber or an STM/AFM probe used therefor. In the terahertz region, however, it has been difficult to use the near-field wave due to the wavelength two or three digits longer than the visible light or the absence of a practical waveguide equivalent to an optical fiber.
The conventional THz imaging using near-field waves described above is roughly divided into an aperture type (Nonpatent Document 1) and an apertureless type (Nonpatent Documents 2 and 3).
In the aperture type, the terahertz wave is focused to a small aperture by using a waveguide or lens and the near-field wave is scattered at the small aperture to detect a near-field wave passing through an object close to the near-field wave by using a detector, by which an image of the object is formed.
In the apertureless type, a probe tip is irradiated with a terahertz wave and a near-field wave is formed at the tip to detect a near-field wave passing through or reflected on the object close to the near-field wave by using a detector, by which an image of the object is formed.
In the conventional apertureless THz imaging using near-field waves, the detector detects a strong far-field wave (an electromagnetic wave propagating in a free space) in addition to a weak near-field wave and therefore a signal-to-noise ratio decreases due to the effect of the far-field wave, which leads to a problem that high-efficiency detection is not achieved.
Moreover, in Nonpatent Document 1 disclosing the aperture THz imaging, the resolution is only one quarter of the wavelength or so and a resolution higher by one digit or more could not be stably obtained, though the near-field wave is used.